1. Field of the Invention
The present invention relates to a method of reducing precipitate formation in a solvent recovery system. The method enables more efficient recovery of the solvent (also referred to as a diluent) reduces formation of solid material in distillation column reboilers and related equipment in the solvent recovery system, and reduces the waste effluent from the system.
2. Description of Related Art
Various types of catalysts useful for polymerizing xcex1-olefins are known. Most of the current catalysts are based on Ziegler-Natta technology, and many of the recent titanium-based olefin polymerization catalysts are stereoregulating, and they have sufficient activity to avoid extraction and deashing. These high activity catalysts typically are prepared from a solid procatalyst that usually contains magnesium, titanium and halide moieties, a cocatalyst (usually an organoaluminum compound) and, when propylene is polymerized in particular, a selectivity control agent (SCA). The solid titanium-containing compound typically is referred to as a xe2x80x9cprocatalyst,xe2x80x9d the organoaluminum compound, whether complexed or not, usually is referred to as the xe2x80x9ccocatalystxe2x80x9d and the third component external electron donor, whether used separately or partially or totally complexed with the organoaluminum compound, is referred to as the xe2x80x9cselectivity control agent.xe2x80x9d Throughout this disclosure, these terms will be used in accordance with the aforementioned designations.
Many chemical combinations of procatalysts, cocatalysts and selectivity control agents are known in the art to produce active catalysts. Through considerable experience, however, certain materials are of greater interest than others. For example, there is significant research in the area of procatalysts, which typically is some chemical combination of magnesium, titanium tetrachloride and an internal electron donor. These internal electron donors usually are aromatic esters such as ethyl benzoate or ethyl p-toluate. Conventional cocatalysts include an aluminum trialkyl such as triethylaluminum or triisobutylaluminum that is often complexed with a portion of the selectivity control agent (or external electron donor), which also is typically an aromatic ester. Although variations in any of these catalyst components will influence the performance of the resultant catalyst, the component that appears to offer the greatest opportunity for modification to produce greater catalyst activity is the procatalyst.
The literature is rife with disclosures relating to the various known methods of preparing procatalysts. For example, Kioka, et al., U.S. Pat. No. 4,330,649, the disclosure of which is incorporated by reference herein in its entirety, describes a solid catalyst component (procatalyst) that is prepared by heating a soluble magnesium compound such as magnesium chloride with a higher alcohol in the presence of an ester to produce a solution. This solution contains a xe2x80x9cprecursorxe2x80x9d of the procatalyst, which then is added to titanium tetrachloride and an electron donor (internal) to form the procatalyst. Brand, U.S. Pat. No. 4,472,521, the disclosure of which is incorporated by reference herein in its entirety, reacts a magnesium alkoxide, wherein each alkoxide has four or more carbons, in the presence of an aromatic hydrocarbon. Titanium tetrachloride and an internal electron donor then are added to the resulting solution to form a solid procatalyst. Arzoumanidis, U.S. Pat. No. 4,540,679, the disclosure of which is incorporated by reference herein in its entirety, produces an olefin polymerization catalyst component by contacting a suspension of magnesium ethoxide in ethanol with carbon dioxide. The addition of organoaluminum in hydrocarbon results in the formation of granular particles that are employed as a support for a titanium compound upon contact with titanium tetrachloride. Nestlerode, et al., U.S. Pat. No. 4,728,705, the disclosure of which is incorporated by reference herein in its entirety, solubilizes magnesium ethoxide in ethanol with carbon dioxide and spray dries the resulting solution or uses the solution to impregnate carrier particles. The solid particles resulting from either modification are useful in the production of a procatalyst of desirable morphology.
A number of United States patents issued to Robert C. Job (and Robert C. Job, et al.,) describe various mechanisms for preparing magnesium-containing, titanium-containing compounds that are useful as precursors for the production of procatalysts that are ultimately useful in preparing catalysts for the polymerization of xcex1-olefins. For example, U.S. Pat. Nos. 5,034,361; 5,082,907; 5,151,399; 5,229,342; 5,106,806; 5,146,028; 5,066,737; and 5,077,357, the disclosures of which are incorporated by reference herein in their entirety, disclose various procatalyst precursors. U.S. Pat. No. 5,034,361 discloses solubilizing a magnesium alkoxide in an alkanol solvent by interaction of the magnesium alkoxide compound and certain acidic materials. This magnesium alkoxide then can be used either directly as a magnesium-containing catalyst precursor, or can be reacted with various titanium compounds to produce a magnesium and titanium-containing catalyst precursor.
U.S. Pat. Nos. 5,082,907; 5,151,399; 5,229,342; 5,106,806; 5,146,028; 5,066,737; and 5,077,357 disclose various magnesium and titanium-containing catalyst precursors, some of which are prepared by using the aforementioned magnesium alkoxide as a starting material. These precursors are not active polymerization catalysts, and they do not contain any effective amounts of electron donor. Rather, the precursors are used as starting materials in a subsequent conversion to an active procatalyst. Magnesium and titanium-containing procatalysts are formed by reacting the magnesium and titanium-containing precursor with a tetravalent titanium halide, an optional hydrocarbon and an electron donor. The resulting procatalyst solid then is separated from the reaction slurry (by filtration, precipitation, crystallization, and the like). These procatalysts are then converted to polymerization catalysts by reaction with, for example, an organoaluminum compound and a selectivity control agent.
Production of these precursors typically involves precipitating a solid magnesium and titanium containing component from solution or suspension and then filtering the suspension containing the precipitated precursor. The solid component can be precipitated by driving off excess alkanol from the solution or suspension, and then the remaining suspension filtered to recover the solid precursor component. The filtrate from the filtration typically contains a number of useful ingredients that can be recovered by various recovery mechanisms. In addition, this composition may include environmentally hazardous ingredients, such as chlorinated hydrocarbons, and the like, Moreover, the reaction diluent used in making the precursor, (e.g., sometimes a chlorinated hydrocarbon), is a valuable ingredient that can be recovered and recycled to the manufacturing unit.
It is known in the art to recover valuable by-products of procatalyst manufacture by subjecting the waste stream to one or more distillations, in the presence or absence of additional solvents, to recover the valuable titanium. U.S. Pat. Nos. 5,242,549 and 5,948,212, the disclosures of which are incorporated by reference in their entirety, both disclose processes of recovering titanium from the waste stream from a procatalyst manufacturing process. These patents are not concerned with recovering an inert reaction diluent from the waste stream of a precursor production unit, nor do they address the problem of preventing the formation of precipitates in a separation unit used to recover valuable by-products from the waste stream of a precursor production unit.
It would be useful to recover the valuable by-products, and to remove environmental hazardous by-products from the waste stream. A known recovery mechanism is to convey the waste stream to a distillation unit whereby the useful and/or hazardous ingredients are removed as light components. Distilling this waste stream, however, can cause precipitation of solid components (e.g., magnesium and titanium containing species), because any remaining alkanol that had dissolved these species is driven off in the subsequent distillation. Precipitation of these species causes undesirable solids accumulation in distillation column reboilers, and other related equipment.
Thus, there exists a need to provide an efficient and effective method of recovering useful ingredients that typically are present in the waste stream from a catalyst precursor manufacturing unit. There also exists a need to provide an efficient and effective method of removing environmentally hazardous ingredients that can be present in the waste stream from a catalyst precursor manufacturing unit. In addition, there exists a need to develop a method of making a catalyst precursor more economically by enabling reuse of valuable by-products or waste effluent, and efficient disposal of waste. There also exists a need to develop a process that prevents precipitation of solid components during distillation of a waste stream from a catalyst precursor production unit, while at the same time enabling efficient recovery of valuable components in the waste stream. It is therefore a feature of an embodiment of the present invention to provide a method of making a catalyst precursor, and a method of recovering and/or removing ingredients from a waste stream from a catalyst precursor production unit, that satisfies these needs, as well as other needs readily apparent to those skilled in the art.
In accordance with these and other features of an embodiment of the present invention, there is provided a method of removing at least one inert reaction diluent and/or wash solvent from a waste stream from a catalyst precursor production unit, the waste stream including at least one reaction diluent and/or wash solvent, at least one titanium alkoxide, at least one magnesium alkoxide, and at least one alkanol. The one or more inert reaction diluent and/or wash solvent can be removed by contacting the waste stream with a solubilization solvent, and then subjecting the resulting stream to distillation. The solubilization solvent: (i) is present in an amount sufficient to maintain the solubility of residual titanium and magnesium alkoxide species; (ii) has a boiling point higher than that of the reaction diluent(s) and/or wash solvent(s); and optionally, but preferably (iii) does not form an azeotrope with the reaction diluent(s) and/or wash solvent(s).
In accordance with an additional feature of an embodiment of the present invention, there is provided a method of making a catalyst precursor that includes contacting at least one titanium alkoxide, at least one magnesium alkoxide, at least one alkanol, and at least one reaction diluent, and removing a portion of the alkanol to precipitate a solid titanium and magnesium containing precursor component, thereby resulting in a suspension of the precursor in the at least one reaction diluent. The solid titanium and magnesium containing precursor component then is separated from the suspension to form a solid precursor component and a waste stream that preferably includes the at least one reaction diluent, at least one titanium alkoxide, at least one magnesium alkoxide, and at least one alkanol. Optionally, the solid precursor component is then subjected to a wash solvent, and then again separated from the suspension to form a solid precursor component and a second or combined waste stream that preferably includes at least one reaction diluent and/or wash solvent, at least one titanium alkoxide, at least one magnesium alkoxide, and at least one alkanol. The one or more inert reaction diluent and/or wash solvent can then be recovered by contacting the waste stream with a solubilization solvent, and then subjecting the resulting stream to distillation. The solubilization solvent: (i) is present in an amount sufficient to maintain the solubility of residual titanium and magnesium alkoxide species; (ii) has a boiling point higher than that of the reaction diluent(s) and/or wash solvent(s); and optionally, but preferably (iii) does not form an azeotrope with the reaction diluent(s) and/or wash solvent(s).
In accordance with yet another feature of an embodiment of the present invention, there is provided a method of making a polymerization catalyst that includes contacting the solid precursor component prepared as above with: (i) an electron donor; (ii) a halide of tetravalent titanium; and (iii) optionally, a hydrocarbon or halohydrocarbon.